@ chrnobel :
I have installed Lazarus version 1.0.8 and compil ide with the main.pp i give in attach.
For me it works like it has to do...

When you have re-compil IDE, close it and reload it, maybe it does not work directly after compil...
Have you enable "Integration into Lazarus" in uidesigner_ext settings ?
You do not have to load uidesigner_ext before loading Lazarus (Open-File in Lazarus will do it..)
What system do you use (Linux 32/64, Windows ?) ?
[EDIT] The uidesigner will be loaded only if the code is fpGUI-type.
So if you load only MyProject.lpi, it will not work.
Have you try to open fpGUITemplate
.lpr or fpGUIDirectTemplate.pas in /uidesigner_ext/template ?